The invention relates to parking brakes for automatic transmissions for automotive vehicles.
Automotive automatic transmissions typically include a hydrokinetic torque converter with a bladed impeller connected to an engine and a turbine connected to gearing. The gearing forms multiple torque flow paths between the turbine and vehicle traction wheels. The transmission may include a transmission park mechanism comprising a toothed wheel connected to a torque output shaft of the transmission and a brake pawl pivoted on the transmission housing.
The pawl has a pawl tooth engageable with tooth spaces on the toothed wheel. This constrains the vehicle against movement under gravity when the vehicle is parked on a sloped surface, for example.
Engagement of the pawl tooth may require slight rotation of the torque output shaft in order to align the pawl tooth with one of the tooth spaces on the toothed wheel. As the pawl drops in place under a spring force, rotation of the output shaft is abruptly locked, sometimes with considerable inertia force impact against a side of an external tooth on the toothed wheel.
The torque-transmitting elements of the powertrain for the vehicle include the transmission, a driveshaft, and the vehicle suspension system. Provision is made for isolating vehicle passengers from noise, vibration and harshness developed by the road wheels by relying on torsional elasticity of these torque-transmitting elements. When the vehicle engine is idling, torque is delivered by the turbine of the automatic transmission torque converter to the traction wheels through multiple-ratio gearing. As the traction wheels are held stationary by wheel brakes, the idling torque of the engine, which is multi plied by the hydrokinetic torque converter, is capable of straining the elastic elements of the powertrain.
When the gear selector mechanism is moved by the vehicle operator out of a forward-driving gear or out of reverse gear into the park mode, torque flow to the traction wheels is removed. This allows strain on the powertrain elements to relax, thereby producing a backlash. The backlash may occur at the instant the park gear tooth engages a tooth space. This often causes the elastic elements in the powertrain to create an objectionable engagement noise in the park mechanism as the pawl tooth engages the side of a tooth of the toothed wheel.
It is an objective of the invention to provide a park gear mechanism for an automatic transmission of the kind described in the preceding discussion whereby elastic damping elements are introduced into the torque flow path as the park mechanism is engaged, thereby decelerating the torque output shaft over a finite angular displacement. This reduces any peak force spike caused by inertia force impact of the park gear with the park pawl as the park mechanism is engaged by the vehicle operator.
An embodiment of the invention includes elastic damping elements, which introduce compliance in the powertrain during engagement of the park gear mechanism. They are located between external projections or drive members on a hub for the park gear mechanism and internal projections or drive members on a park gear toothed wheel. Torsional forces in the powertrain deform the compliant elastic damping elements, which are sized to absorb typical driveshaft unwind occurrences within the limits of the design deformation.
The improved damper mechanism is capable of withstanding maximum torsional forces imposed by a parked vehicle as hard stops on the park gear hub and on the surrounding toothed wheel are engaged at the limit of the design deformation of the compliant elements.
The toothed wheel surrounds the park gear hub, which is secured to the transmission driven shaft. It has external park gear teeth on its periphery. The external drive members extend radially outward from the park gear hub.
The internal drive members extend radially inward from the toothed wheel. A park pawl is pivotally mounted on a housing portion of the transmission mechanism, the pawl having a pawl tooth engageable with a space between two adjacent park gear teeth, thereby locking the driveshaft stationary in a vehicle park mode.
The elastic elements are located between adjacent surfaces of the internal drive members and the external drive members whereby torsional inertia forces are cushioned as the pawl tooth enters a park gear tooth space during the vehicle park mode. Gaps between at least one external drive member and adjacent internal drive member provide deformation limits for the elastic elements.